Polyethylene (PE) is synthesised via polymerising ethylene (CH2═CH2) monomers. Because PE is cheap, safe, stable to most environments and easily processed, polyethylene polymers are useful in many applications. According to its properties polyethylene can be classified into several types, such as but not limited to LDPE (Low Density Polyethylene), LLDPE (Linear Low Density Polyethylene), and HDPE (High Density Polyethylene). Each type of polyethylene has different properties and characteristics.
It is known to produce polyethylene from ethylene monomer in the presence of diluent and catalyst and optionally one or more co-monomers and molecular weight regulators in a loop reactor. Usually the loop reactor is a liquid phase loop reactor wherein the components are circulated under pressure in slurry conditions. The product usually consists of solid particles and is in suspension in a diluent. The liquid diluent can be any hydrocarbon inert and liquid during ethylene polymerisation, for example alkanes, such as isobutane. The catalyst for producing polyethylene may typically comprise a chromium-based catalyst, a Ziegler-Natta catalyst or a metallocene catalyst. Molecular weight regulator is usually hydrogen, if added. A co-monomer can be any alpha-olefin with at least three carbons.
Continuous circulation of the slurry contents of the reactor is maintained with a pump, which also ensures efficient suspension of the polymer solid particles in the liquid diluent. Circulation is carried out at elevated polymerisation temperatures around the loop reactor, thereby producing polyethylene.
The product is discharged by means of settling legs, which operate on a batch principle to recover the product. Settling in the legs is used to increase the concentration of solids in the slurry to be recovered as product slurry. The product is further discharged to a flash tank, through flash lines, where most of the diluent and unreacted monomers are flashed off and recycled. The polymer particles are dried, additives can be added and finally the polymer is extruded and pelletised.
Alternatively, after discharging the product slurry from the settling legs, the reaction mixture may be fed to a second loop reactor serially connected to the first loop reactor where a second polyethylene fraction may be produced. Typically, when two reactors in series are employed in this manner, i.e. a first polyethylene fraction produced in a first reactor and a second polyethylene fraction produced in a second reactor, the resultant polyethylene product has a broad or bimodal molecular weight distribution.
Ethylene co-polymerisation is the process wherein ethylene is polymerised with co-monomer, i.e. an alpha-olefin, such as e.g. propylene, butene, hexene, etc. The lower the desired density of the final polyethylene, the higher the concentration of co-monomer in the reactor must be. A major problem in such co-polymerisation processes is that the control of reaction parameters is very difficult. In particular, the ratio of co-monomer to ethylene monomer differs at different points in the reactor. It is also becomes more difficult to control and optimise reaction conditions, such as reaction temperatures, when producing linear low-density polyethylene.
The operating temperature in the reactor has to be set as high as possible in order to have optimum conditions i.e. the higher the temperature in the reactor, the higher the productivity of the catalyst. However, increasing the temperature also increases the risk of swelling occurring in the reactor. Swelling is a phenomenon that occurs when diluent, e.g. isobutane, enters the polymer fluff and dissolves low molecular weight polymers. Comonomer, e.g. hexene, if present, is an even better solvent for low molecular weight polymers. Hence co-polymerisations suffer from a higher risk of swelling than homo-polymerisations. Swelling is also the result of the absorption of diluent and comonomer into the polymer grains. Consequently, the polymer slurry becomes more viscous, which perturbs the reactor flow and may even lead to blockage of the reactor. Therefore, temperature and slurry density must be well controlled in order to avoid the solubility of the lighter polymer fractions in the diluent. Solubility increases with increasing co-monomer concentration and with increasing temperature.
In chromium- and Ziegler-Natta-catalysed ethylene polymerisations, the onset of swelling can be detected by observing an increase in the power consumption of the circulation pump and also by an increase in the density of the slurry using the “x-ray sensor 3” (Rx) detector. As the reaction medium begins to become more viscose, the pump requires more power (kW) for circulating the slurry. The warning signal for swelling is thus an increase in the amplitude of the kW variation of the pump. When this is observed, the reaction medium can be quickly diluted with more diluent, e.g. isobutane, to return the slurry density to more optimal values. However, quick addition of diluent leads to a temporary loss in control over polymer properties, thereby resulting in non-homogeneous properties in the final polymer product.
Furthermore, these early warning signals are absent during metallocene-catalysed polymerisations. This is because metallocene-catalysed polyethylene has a much narrower molecular weight distribution (MWD). This means there is almost no tailing of low molecular weight compounds and far less which thus could dissolve into the diluent or comonomer and provide a warning signal for swelling. In addition metallocenes are single-site catalysts, which means that the growing polymer chains are always all roughly the same length. Furthermore, the weight average molecular weight (MW) of metallocene-catalysed polyethylene is usually smaller than the MW of chromium-catalysed polyethylene. Since the growing polymer chains are all shorter and all of similar length, once conditions are suitable, i.e. the temperature is high enough, almost all of the polymer will immediately dissolve into the reaction medium simultaneously. The slurry becomes more viscous and thus more difficult to circulate in the loop reactor. Immediate swelling may also lead to sudden blockage of the reactor.
In the past, the risk of swelling was decreased by setting the polymerisation temperature well below the temperature at which swelling is believed to pose a problem. Classically, this temperature has been predicted for chromium-catalysed polymerisations by calculating it from the linear relationship between the reaction temperature and the resin density i.e. the swelling curve. However, this swelling curve does not take into account the actual co-monomer concentrations in the reactor, nor does it take into account the molecular weight of the desired polyethylene. It also does not provide a method of incorporating MWD effects. Furthermore, these calculations are not applicable to metallocene-catalysed polymerisations. Metallocenes behave differently in response to co-monomer concentrations. To produce a resin of equivalent density, a metallocene-based resin requires a lower co-monomer concentration than a chromium-based resin, as it is much more efficient at co-monomer incorporation. The MWD of metallocene-produced resins are also much narrower.
The problem with using traditional swelling curves is that they do not allow the full potential of the catalyst to be exploited. Actual operating temperatures are usually far below the optimum temperatures, which can be utilised without the risk of swelling. As a result of the low reactor temperatures, the catalyst has limited productivity, the polymer has difficulties settling in the settling legs and co-monomer is not incorporated efficiently. Furthermore, especially chromium-catalysed polyethylenes show much lower melt potentials when polymerisation temperatures are too low.
The problem of swelling is particularly acute for metallocene-catalysed polymerisations due to the lack of warning signals.
In view hereof, there is a need in the art to provide a process for improving the polymerisation reaction of ethylene and in particular with α-olefin co-monomers, such that the co-polymerisation process is optimised and more homogeneous polymer end products are obtained.
It is therefore an aim of the invention to optimise the ethylene slurry polymerisation process.
It is also an aim to increase the polymerisation temperature during an ethylene slurry polymerisation process.
It is further an aim of the invention to decrease the risk of swelling in an ethylene slurry polymerisation process.
It is still another aim of the invention to increase the productivity of catalysts in ethylene slurry polymerisations.
Furthermore, it is an aim of the invention to increase the melt potential of polyethylenes produced in slurry reactors.
It is an additional aim of the invention to increase the efficiency of hexene incorporation into ethylene copolymers produced in slurry reactors.
Further to these aims, it is also an aim of the invention to increase the settling efficiency and decantation of the product slurry.
At least one of these aims is fulfilled by the current invention.